This project is intended to evaluate the safety and efficacy of three doses of mazapertine (20, 60, and 120 mg. bid), compared with risperidone (3 mg. bid) and placebo, in the treatment of patients with chronic schizophrenia. Specific objectives of this study are to: (1) Determine the optimal dose of mazapertine in chronic schizophrenic subjects, compared to placebo and to the optimal dose of risperidone; (2) Explore the effects on psychopathological aspects of schizophrenia other than psychosis, such as mood and anxiety disturbance; and (3) Evaluate the dose-linearity of mazapertine pharmacokinetics and the prolactin response to the drug in a subgroup of subjects.